Refactor and modernize namespace declarations

 * Completely remove inline namespace v2
 * Flatten some unnecessarily deep namespace nesting
 * Move DummyClientFace, Segmenter, SegmentFetcher to namespace ndn
 * Move all unit tests to namespace ndn::tests

Change-Id: I8bcfcf9fd669936a3277d2d5d505f765b4b05742
diff --git a/tools/ndnsec/list.cpp b/tools/ndnsec/list.cpp
index 502d909..3b02b80 100644
--- a/tools/ndnsec/list.cpp
+++ b/tools/ndnsec/list.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2022 Regents of the University of California.
+ * Copyright (c) 2013-2023 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -25,8 +25,7 @@
 
 #include "ndn-cxx/util/indented-stream.hpp"
 
-namespace ndn {
-namespace ndnsec {
+namespace ndn::ndnsec {
 namespace {
 
 // https://en.wikipedia.org/wiki/Box_Drawing
@@ -247,5 +246,4 @@
   return 0;
 }
 
-} // namespace ndnsec
-} // namespace ndn
+} // namespace ndn::ndnsec